The present invention concerns an optoelectronic sensor and the method for monitoring a three-dimensional space along a plurality of planes.
The automated monitoring of a three-dimensional space has many applications. For example, it may be used to prevent theft, in the automation industry, as well as for security purposes. In connection with the operation and/or securing of potentially dangerous machines, it is especially necessary to detect the presence of an object or a body part, often including the relative position thereof in a three-dimensional space.
The prior art includes a variety of proposals for monitoring three-dimensional spaces. However, each of the prior art “solutions” has shortcomings which limit its usefulness.
For example, it is known to monitor a space with a camera and to compare the picture taken by the camera with a reference picture in order to detect changes in the three-dimensional space, such as the entry of an object into the space, and to react accordingly. A further development of such technology is to take three-dimensional pictures. This requires a double camera and a stereoscopic evaluation thereof, as is disclosed for example in U.S. Pat. No. 6,297,844 B1.
In addition to relatively extensive calculations that must be performed in connection with such a stereoscopic method, this approach has the further disadvantage that all monitoring originates from a single point. This results in a pyramid or cone-like monitored region or measurement field. A rectangularly-shaped space within such a monitored region creates significant evaluation problems. In addition, objects, such as pallets, measuring heads and the like, can be in or can extend into the monitored region. The sight shadow created by such objects creates a non-monitored space.
Another approach is disclosed in DE 197 57 848 C2. It employs a laser scanner to direct its light via a system of mirrors and apertures along several planes. One particular embodiment consists of several scanning lasers arranged next to each other and so that a mirror can be moved to generate a plurality of parallel, spaced-apart scanning planes. A disadvantage of this approach is that it requires mechanical components, such as the rotating scanning laser, which are costly and subject to failures. In addition, the planes must be closely adjacent to each other because the several scanning lasers would otherwise be heavy and the resulting inertia would make rapid rotational movements difficult or impossible. Monitoring a space which extends beyond the immediate vicinity of a plane is therefore not possible with this approach.
It is further known to place a type of cage defined by light grids or scanners about a machine that is to be secured. In such a case, light columns are placed at the four corners of a square or rectangular base surface so that its side surfaces, for example, can be illuminated. An obvious disadvantage of this configuration is that several light columns are needed. They interfere with normal operations around the protected machine and create relatively high costs.
Finally, a number of approaches monitor a single plane. This can be a light grid, a laser scanner or a triangulation sensor with a camera element. It is further feasible to include a distance-measuring picture or video sensor. Of course, such sensors can only monitor a surface and not a three-dimensional space.